Resistive Switching Properties Of Epitaxial NiO_x Films On Metal Seed Layers

Resistive switching random access memory(Re RAM) based on resistive switching(RS) of oxide films is one of potential candidates for next generation of nonvolatile memory devices due to its high writing/read speed, simple structure and low power assumption. NiO_x is one of the most promising oxide materials applied for commercial Re RAMs. However, NiO_x films present diverse PS behaviors which are heavily dependent on fabrication technologies and conditions. Up to now, RS performances and mechanisms of polycrystalline NiO_x films have been extensively investigated, while discussions on RS performances of epitaxial NiO_x films, especially on metal seed layer are relatively rare. Platinum exhibits excellent electrical and thermal conductivity, high anti-corrosion and good chemical stability, which is usually used to be seed/buffer layers or bottom electrodes for the growth of epitaxial perovskite oxide films, because it has a closed-packed face-centered cubic lattice structure with a lattice constant(a = 0.3923 nm) matching with most of the perovskite materials.In this paper, highly textured Pt films are selected to be the seed layers for growing epitaxial NiO_x films on it. Their microstructures and resistance switching characteristics have been investigated.Epitaxial Pt films have been deposited by magnetron sputtering at 700℃ on Mg O and α-Al2O3 singal-crystal substrates respectively. Effects of substrate’s texture and deposition ambient on the crystal orientation and surface morphology of Pt films have been studied. Experimental results indicate that the growth of epitaxial Pt(111) films are only determined by the lattice structure of substrate, and its surface morphology is determined by deposition ambient, film thickness and annealing treatments. On the contrary, the growth of epitaxial Pt(100) thin films is determined by both the lattice structure of substrate and the deposition ambient. X-ray diffraction investigations reveal the epitaxial growth of Pt(100)// Mg O(100), Pt(111)//Mg O(111), Pt(111) //α-Al2O3(0001) and Pt(111)//α-Al2O3(01 12). Scanning electron microscopy observations reveal epitaxial Pt(111) films on Mg O(111),α-Al2O3(0001) and(01 12) substrates deposited in mixed ambient with a O2-Ar ratio of 15% exhibit smooth and dense morphology assembled by triangle crystal grains, while epitaxial Pt(100) films on Mg O(100) substrate deposited under same conditions show multilayer structures embedded with random microscale pinholes. The size of pinholes increases with the increase of oxygen partial pressure, which results in the increase of surface roughness of Pt(100) films. On the other hand, measured by using four probe method, the resistivity of epitaxial Pt(100) films increases gradually with the increase of oxygen partial pressure, but still maintains good electrical conductivity.After selecting epitaxial Pt(100) films on Mg O(100) substrates as seed layers, NiO_x films have been deposted on it under different conditions. X-ray diffraction investigations reveal that NiO_x films deposited at lower than 480℃ substrate temperature are amorphous, and epitaxial NiO_x(111) films are only obtained when the substrate temperature is higher than 480℃. After annealing at 680℃ in Ar ambient for 30 min, amorphous NiO_x films deposited at 280℃ have crystallized to highly textured NiO_x(100) films. Scanning electron microscopy observations reveal amorphous NiO_x films exhibit a smooth and dense surface morphology, which is composed by uniform particles with a average size of 10 nm; epitaxial NiO_x(111) films exhibit a smooth and dense surface composed by triangular crystalline grains; while epitaxial NiO_x(100) films show the similar multilayer structures embedded with random microscale pinholes with the Pt(100) seed layer. Current-voltage measurements indicate that NiO_x(111) films possess a bipolar RS pattern with a counterclockwise cycle direction: the ratio of high/low resistance(RH/RL) is 40,which can be well maintained for 300 cycles. NiO_x(100) films also possess similar RS characteristics: the RH/RL is 37,which can be well maintained for 430 cycles; On the contrary, amorphous NiO_x films possess both threshold-type RS and bipolar RS characteristics, the biggest RH/RL is 70, which can last 600 cycles. After inserting the Hf O2 buffer layer between Ag top electrodes and amorphous NiO_x films, the RH/RL abruptly increases to 596, but the cycle number decreases to 30. It is argued that the diversity of RS characteristics for NiO_x films on metal seed layer is attributed to the cooperation and competition between conductive bridge channels composed by Ag ions diffused from Ag top electrodes into NiO_x film and conductive filamentaries composed by drifted oxygen vacancies driven by the periodic electric field.In summary, epitaxial NiO_x films on metal base show impressive RS characteristics, which is different from normal polycrystalline NiO_x films, have potential applications in next generation of Re RAM devices.